Surfaces, Clean and Contaminated. 



395 



If we take as the reading when the camphor is nearly dead 

 15*2, we find in like manner 



T=62-7-9-l = 53-6. 



After this example a summary of results may suffice. The 

 interest attaching to the determination of the tension of a 

 clean surface led me to strive after a higher degree of accuracy 

 than perhaps would otherwise have been necessary. The 

 following table contains the results obtained with both forks: — 



Date. 



Distance. 



Frequency. 



Tension. 



Water. 



Temp. F. 



Remarks. 



1890. 











o 





June 23... 



905 



40-9 



72-3 



Tap 







25... 



4-12 



409 



74-5 



j> 



"73" 



Telescope 



26... 



11-70 



40-9 



73-7 



j> 



66 



lens. 



30... 



11-27 



40-12 



740 



>> 



61 





July 1... 



9-96 



42-12 



73-2 



)> 



61 





2. . 



9-90 



42-12 



74-7 





62 



Strip dipper 



4... 



9-96 



42-12 



74-7 



Distilled 



64 



introduced. 



8... 



9-96 



42-12 



74-7 



» 



60 





25... 



10-00 



42-12 



74-2 



Tap 



65 





25... 



9-20 



42-12 



75-2 



5> 



65 





28... 



9-20 



4212 



743 



>J 



68 





28... 



9-20 



42-12 



743 



Distilled 



68 





29... 



9-20 



42-12 



74-3 



j> 



, .... 





Aug. 2... 



9-20 



4212 



74-3 



Tap 



63 





July 23... 



10-00 



124-9 



741 



Tap 



65 





23... 



9-49 



124-9 



73-2 



}J 



66 





23... 



8-13 



124-9 



73-5 



Distilled 



66 





The mean result with the graver fork is T=74'2 ; and 

 with the quicker one T = 73*6. The discrepancy of nearly 

 one per cent, marks the limit of accuracy. It should be re- 

 marked that some of the consecutive results where no varia- 

 tion occurred in the distance between the points cannot be 

 regarded as quite independent. 



On several occasions distilled water proved a less satis- 

 factory subject than tap water. The surface seemed more 

 unwilling to become and remain clean. Sometimes after ex- 

 pansion a notable increase of readings would occur in the 

 course of a few minutes without assignable cause. 



I was very anxious to satisfy myself that in the surfaces 

 experimented upon by the wave method a high degree of 

 purity was really attained. In the experiments of July 28 a 

 Plateau needle vibrating upon a portable stand was introduced. 

 After the examination by the method of waves the dish was 

 brought out into a good light, and the quality of the surface 

 tested by observation of the motion of motes when the needle 



